Organizing Code in Web Apps. SWE 432, Fall 2016 Design and Implementation of Software for the Web

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1 Organizing Code in Web Apps SWE 432, Fall 2016 Design and Implementation of Software for the Web

2 Today Some basics on how and why to organize code (SWE!) Closures Classes Modules For further reading: 2

3 History + Motivation Wow back in my day before ES6 we didn t have your fancy modules

4 Spaghetti Code Brian Foote and Joe Yoder 4

5 window.onload = function () { eqctl = document.getelementbyid('eq'); currnumberctl = document.getelementbyid('currnumber'); ; var eqctl, currnumberctl, operator, operatorset = false, equalspressed = false, lastnumber = null; function add(x,y) { return x + y; function subtract(x, y) { return x - y; function multiply(x, y) { return x * y; function divide(x, y) { if (y == 0) { alert("can't divide by 0"); return 0; return x / y; function setval(val) { currnumberctl.innerhtml = val; function setequation(val) { eqctl.innerhtml = val; function clearnumbers() { lastnumber = null; equalspressed = operatorset = false; setval('0'); setequation(''); function setoperator(newoperator) { if (newoperator == '=') { equalspressed = true; calculate(); setequation(''); return; if (!equalspressed) calculate(); equalspressed = false; operator = newoperator; operatorset = true; lastnumber = parsefloat(currnumberctl.innerhtml); var eqtext = (eqctl.innerhtml == '')? lastnumber + ' ' + operator + ' ' : eqctl.innerhtml + ' ' + operator + ' '; setequation(eqtext); function numberclick(e) { var button = (e.target)? e.target : e.srcelement; if (operatorset == true currnumberctl.innerhtml == '0') { setval(''); operatorset = false; setval(currnumberctl.innerhtml + button.innerhtml); setequation(eqctl.innerhtml + button.innerhtml); function calculate() { if (!operator lastnumber == null) return; var currnumber = parsefloat(currnumberctl.innerhtml), function setoperator(newoperator) { if (newoperator == '=') { equalspressed = true; calculate(); setequation(''); return; if (!equalspressed) calculate(); equalspressed = false; operator = newoperator; operatorset = true; lastnumber = parsefloat(currnumberctl.innerhtml); var eqtext = (eqctl.innerhtml == '')? lastnumber + ' ' + operator + ' ' : eqctl.innerhtml + ' ' + operator + ' '; setequation(eqtext); function numberclick(e) { var button = (e.target)? e.target : e.srcelement; if (operatorset == true currnumberctl.innerhtml == ' setval(''); operatorset = false; setval(currnumberctl.innerhtml + button.innerhtml); setequation(eqctl.innerhtml + button.innerhtml); function calculate() { if (!operator lastnumber == null) return; var currnumber = parsefloat(currnumberctl.innerhtml), newval = 0; switch (operator) { case '+': newval = add(lastnumber, currnumber); break; newval = 0; case '-': switch (operator) { case '+': newval = add(lastnumber, currnumber); newval = subtract(lastnumber, currnumber); break; case '-': break; newval = subtract(lastnumber, currnumber); break; case '*': case '*': newval = multiply(lastnumber, currnumber); break; newval = multiply(lastnumber, currnumber); case '/': newval = divide(lastnumber, currnumber); break; break; case '/': setval(newval); lastnumber = newval; newval = divide(lastnumber, currnumber); break; setval(newval); 5

6 aka big ball of mud aka shanty town code Brian Foote and Joe Yoder 6

7 Bad Code Smells Tons of not-very related functions in the same file No/bad comments Hard to understand Lots of nested functions fs.readdir(source, function (err, files) { if (err) { console.log('error finding files: ' + err) else { files.foreach(function (filename, fileindex) { console.log(filename) gm(source + filename).size(function (err, values) { if (err) { console.log('error identifying file size: ' + err) else { console.log(filename + ' : ' + values) aspect = (values.width / values.height) widths.foreach(function (width, widthindex) { height = Math.round(width / aspect) console.log('resizing ' + filename + 'to ' + height + this.resize(width, height).write(dest + 'w' + width + if (err) console.log ).bind(this)) ) ) ); 7

8 Design Goals Within a component Cohesive Complete Convenient Clear Consistent Between components Low coupling 8

9 Cohesion and Coupling Cohesion is a property or characteristic of an individual unit Coupling is a property of a collection of units High cohesion GOOD, high coupling BAD Design for change: Reduce interdependency (coupling): You don't want a change in one unit to ripple throughout your system Group functionality (cohesion): Easier to find things, intuitive metaphor aids understanding 9

10 Design for Reuse Why? Don t duplicate existing functionality Avoid repeated effort How? Make it easy to extract a single component: Low coupling between components Have high cohesion within a component 10

11 Design for Change Why? Want to be able to add new features Want to be able to easily maintain existing software Adapt to new environments Support new configurations How? Low coupling - prevents unintended side effects High cohesion - easier to find things 11

12 Organizing Code How do we structure things to achieve good organization? Java Javascript Individual Pieces of Functional Components Classes Classes Entire libraries Packages Modules 12

13 Closures Closures are expressions that work with variables in a specific context Closures contain a function, and its needed state Closure is that function and a stack frame that is allocated when a function starts executing and not freed after the function returns 13

14 Closures & Stack Frames What is a stack frame? Variables created by function in its execution Maintained by environment executing code function a() { var x = 5, z = 3; b(x); function b(y) { console.log(y); a(); Contents of memory: a: x: 5 z: 3 Stack frame Function called: stack frame created 14

15 Closures & Stack Frames What is a stack frame? Variables created by function in its execution Maintained by environment executing code function a() { var x = 5, z = 3; b(x); function b(y) { console.log(y); a(); Contents of memory: b: y: 5 a: x: 5 z: 3 Stack frame Function called: new stack frame created 15

16 Closures & Stack Frames What is a stack frame? Variables created by function in its execution Maintained by environment executing code function a() { var x = 5, z = 3; b(x); function b(y) { console.log(y); a(); Contents of memory: a: x: 5 z: 3 Stack frame Function returned: stack frame popped 16

17 Closures Closures are expressions that work with variables in a specific context Closures contain a function, and its needed state Closure is a stack frame that is allocated when a function starts executing and not freed after the function returns That state just refers to that state by name (sees updates) var x = 1; function f() { var y = 2; return function() { console.log(x + y); y++; ; var g = f(); g(); // 1+2 is 3 g(); // 1+3 is 4 This function attaches itself to x and y so that it can continue to access them. It closes up those references 17

18 Closures var x = 1; function f() { var y = 2; return function() { console.log(x + y); y++; ; var g = f(); g(); // 1+2 is 3 g(); // 1+3 is 4 f() Global var x 1 var y function 2 Closure 18

19 Closures var x = 1; function f() { var y = 2; return function() { console.log(x + y); y++; ; var g = f(); g(); // 1+2 is 3 g(); // 1+3 is 4 Global var x 1 f() var y 3 Closure function 19

20 Closures var x = 1; function f() { var y = 2; return function() { console.log(x + y); y++; ; var g = f(); g(); // 1+2 is 3 g(); // 1+3 is 4 f() Global var x 1 var y 4 Closure function 20

21 Modules We can do it with closures! Define a function Variables/functions defined in that function are private Return an object - every member of that object is public! Remember: Closures have access to the outer function s variables even after it returns 21

22 Modules with Closures var facultyapi = (function(){ var faculty = [{name:"prof Bell", section: 2, {name:"prof LaToza", section:1]; return { getfaculty : function(i) { return faculty[i].name + " ("+faculty[i].section +")"; ; )(); console.log(facultyapi.getfaculty(0)); This works because inner functions have visibility to all variables of outer functions! 22

23 Closures gone awry var funcs = []; for (var i = 0; i < 5; i++) { funcs[i] = function() { return i; ; What is the output of funcs[0]()? >5 Why? Closures retain a pointer to their needed state! 23

24 Closures under control Solution: IIFE - Immediately-Invoked Function Expression function makefunction(n) { return function(){ return n; ; for (var i = 0; i < 5; i++) { funcs[i] = makefunction(i); Why does it work? Each time the anonymous function is called, it will create a new variable n, rather than reusing the same variable i Shortcut syntax: var funcs = []; for (var i = 0; i < 5; i++) { funcs[i] = (function(n) { return function() { return n; )(i); 24

25 Exercise: Closures var facultyapi = (function(){ var faculty = [{name:"prof Bell", section: 2, {name:"prof LaToza", section:1]; return { getfaculty : function(i) { return faculty[i].name + " ("+faculty[i].section +")"; ; )(); console.log(facultyapi.getfaculty(0)); Here s our simple closure. Add a new function to create a new faculty, then call getfaculty to view their formatted name. 25

26 Classes A small correction: * Lecture 4, JavaScript 26

27 Classes ES6 introduces the class keyword Mainly just syntax - still not like Java Classes Old New function Faculty(first, last, teaches, office) { this.firstname = first; this.lastname = last; this.teaches = teaches; this.office = office; this.fullname = function(){ return this.firstname + " " + this.lastname; var profjon = new Faculty("Jonathan", "Bell", "SWE432", "ENGR 4322"); class Faculty { constructor(first, last, teaches, office) { this.firstname = first; this.lastname = last; this.teaches = teaches; this.office = office; fullname() { return this.firstname + " " + this.lastname; var profjon = new Faculty("Jonathan", "Bell", "SWE432", "ENGR 4322"); 27

28 Classes - Extends extends allows an object created by a class to be linked to a super class. Can (but don t have to) add parent constructor. class Faculty { constructor(first, last, teaches, office) { this.firstname = first; this.lastname = last; this.teaches = teaches; this.office = office; fullname() { return this.firstname + " " + this.lastname; class CoolFaculty extends Faculty { fullname() { return "The really cool " + super.fullname(); 28

29 Classes - static static declarations in a class work like in Java class Faculty { constructor(first, last, teaches, office) { this.firstname = first; this.lastname = last; this.teaches = teaches; this.office = office; fullname() { return this.firstname + " " + this.lastname; static formatfacultyname(f) { return f.firstname + " " + f.lastname; 29

30 Modules (ES6) With ES6, there is finally language support for modules Module must be defined in its own JS file Modules export declarations Publicly exposes functions as part of module interface Code imports modules (and optionally only parts of them) Specify module by path to the file 30

31 Modules (ES6) - Export Syntax var faculty = [{name:"prof Bell", section: 2, {name:"prof LaToza", section:1]; export function getfaculty(i) { Label each declaration with //.. export export var somevar = [1,2,3]; var faculty = [{name:"prof Bell", section: 2, {name:"prof LaToza", section:1]; var somevar = [1,2,3]; function getfaculty(i) { //.. export {getfaculty, somevar; Or name all of the exports at once export {getfaculty as aliasforfunction, somevar; Can rename exports too export default function getfaculty(i){... Default export 31

32 Modules (ES6) - Import Syntax Import specific exports, binding them to the same name import { getfaculty, somevar from "mymodule"; getfaculty()... Import specific exports, binding them to a new name import { getfaculty as aliasforfaculty from "mymodule"; aliasforfaculty()... Import default export, binding to specified name import thething from "mymodule"; thething()... -> calls getfaculty() Import all exports, binding to specified name import * as facmodule from "mymodule"; facmodule.getfaculty()... 32

33 Patterns for using/creating libraries Try to reuse as much as possible! Name your module in all lower case, with hyphens Include: README.md keywords, description, and license in package.json (from npm init) Strive for high cohesion, low coupling Separate models from views How much code to put in a single module? Cascades (see jquery) 33

34 Cascade Pattern aka chaining Offer set of operations that mutate object and returns the this object Build an API that has single purpose operations that can be combined easily Lets us read code like a sentence Example (String): str.replace("k","r").touppercase().substr(0,4); Example (jquery): $( #wrapper").fadeout().html( Welcome").fadeIn(); 34

35 Demo: Modules Not yet supported by any browser! 35

36 Closures Exercise Work from our example before of the Faculty Closure API to create a Class API (with Closures). Private fields: Faculty API List of students (students are objects with names, section numbers, and partners [which are students]) Public functions: Add a student to the class Retrieve the name of the student s faculty

37 Exit-Ticket Activity Go to socrative.com and select Student Login Class: SWE (Prof LaToza) or SWE (Prof Bell) ID is 1: How well did you understand today's material 2: What did you learn in today's class? For question 3: What happens when the user clicks on the 4th button on this page and why? var nodes = document.getelementsbytagname('button'); for (var i = 0; i < nodes.length; i++) { nodes[i].addeventlistener('click', function() { console.log('you clicked element #' + i); );